Microbes are tiny organisms that are too tiny to see without a microscope. They live everywhere—in air, soil, rock, and water. Some microbes cause disease in humans, plants, and animals. Others are essential for a healthy life, and we could not exist without them. The relationship between microbes and humans is delicate and complex.

Most microbes belong to one of four major groups: bacteria, viruses, fungi, or protozoa. Those that cause disease might be referred to simply as "germs."

Different Kinds of Infections

Some disease-causing microbes can make you very sick quickly and then not bother you again. Some can last for a long time and continue to damage tissues. Others can last forever, but you won’t feel sick anymore, or you will feel sick only once in a while. Most infections caused by microbes fall into three major groups:

Acute infections

Chronic infections

Latent infections

Acute Infections

Acute infections are usually severe and last a short time. They can make you feel very uncomfortable, with signs and symptoms such as tiredness, achiness, coughing, and sneezing. The common cold is such an infection. The signs and symptoms of a cold can last for 2 to 24 days (but usually a week), though it may seem like a lot longer. Once your body’s immune system has successfully fought off one of the many different types of rhinoviruses or other viruses that may have caused your cold, the cold doesn’t come back. If you get another cold, it’s probably because you have been infected with other cold-causing viruses.

Chronic Infections

Chronic infections usually develop from acute infections and can last for days to months to a lifetime. Sometimes people are unaware they are infected but still may be able to transmit the germ to others. For example, hepatitis C, which affects the liver, is a chronic viral infection. In fact, most people who have been infected with the hepatitis C virus don’t know it until they have a blood test that shows antibodies to the virus. Recovery from this infection is rare — about 85 percent of infected people become chronic carriers of the virus. In addition, serious signs of liver damage, like cirrhosis or cancer, may not appear until as long as 20 years after the infection began.

Latent Infections

Latent infections are “hidden” or “silent” and may or may not cause symptoms again after the first acute episode. Some infectious microbes, usually viruses, can “wake up”—become active again but not always causing symptoms—off and on for months or years. When these microbes are active in your body, you can transmit them to other people. Herpes simplex viruses, which cause genital herpes and cold sores, can remain latent in nerve cells for short or long periods of time, or forever.

Chickenpox is another example of a latent infection. Before the chickenpox vaccine became available in the 1990s, most children in the United States got chickenpox. After the first acute episode, usually when children are very young, the Varicella zoster virus goes into hiding in the body. In many people, it emerges many years later when they are older adults and causes a painful disease of the nerves called herpes zoster, or shingles.

Transmission of Infectious Diseases

Some microbes can travel through the air

You can transmit microbes to another person through the air by coughing or sneezing. These are common ways to get viruses that cause colds or flu, or the bacteria that cause tuberculosis. Interestingly, international airplane travel can expose you to germs not common in your own country.

Close contact can pass germs to another person

Scientists have identified more than 500 types of bacteria that live in our mouths. Some keep the oral environment healthy, while others cause problems like gum disease. One way you can transmit oral bacteria is by kissing.

Microbes such as HIV, herpes simplex virus type 2, which causes genital herpes, and Neisseria gonorrhoeae bacteria, which causes gonorrhea, are examples of germs that cyou can pass to another during sexual intercourse.

You can pick up and spread germs by touching infectious material

A common way for some microbes to enter the body, especially when caring for young children, is through unintentionally passing feces from hand to mouth or the mouths of young children. Infant diarrhea is often spread in this way. Daycare workers, for example, can pass diarrhea-causing rotavirus or Giardia lamblia (protozoa) from one baby to the next between diaper changes and other childcare practices.

It also is possible to pick up cold viruses from shaking someone’s hand or from touching contaminated surfaces, such as a doorknob or computer keyboard.

A healthy person can carry germs and pass them onto others

The story of “Typhoid Mary” is a famous example from medical history about how a person can pass germs on to others, yet not be affected by those germs. The germs in this case were Salmonella typhi bacteria, which cause typhoid fever and are usually spread through food or water.

Germs from your household pet can make you sick

You can catch a variety of germs from animals, especially household pets. The rabies virus, which can infect cats and dogs, is one of the most serious and deadly of these microbes. Fortunately, rabies vaccine prevents animals from getting rabies. Vaccines also protect people from accidentally getting the virus from an animal. In addition, vaccines prevent people who already have been exposed to the virus, such as through an animal bite, from getting sick.

Dog and cat saliva can contain any of more than 100 different germs that can make you sick. Pasteurella bacteria, the most common, can be transmitted through bites that break the skin causing serious, and sometimes fatal, diseases such as meningitis. Meningitis is the inflammation of the lining of the brain and spinal cord.

Warm-blooded animals are not the only ones that can cause you harm. Pet reptiles such as turtles, snakes, and iguanas can give Salmonella bacteria to their unsuspecting owners.

You can get microbes from tiny critters

Mosquitoes may be the most common carriers, also called vectors, of pathogens. Anopheles mosquitoes can pick up Plasmodium, which causes malaria, from the blood of an infected person and transmit the protozoan to an uninfected person.

Fleas that pick up Yersinia pestis bacteria from rodents can then transmit plague to humans.

Ticks, which are more closely related to crabs than to insects, are another common vector. The tiny deer tick can infect humans with Borrelia burgdorferi, the bacterium that causes Lyme disease, which the tick picks up from mice.

Some microbes in food or water could make you sick

Every year, millions of people worldwide become ill from eating contaminated foods. Although many cases of foodborne illness or “food poisoning” are not reported, the Centers for Disease Control and Prevention (CDC) estimates there are 76 million cases of such illnesses in the United States each year. In addition, CDC estimates 325,000 hospitalizations and 5,000 deaths are related to foodborne diseases each year. Microbes can cause these illnesses, some of which can be fatal if not treated properly.

Poor manufacturing processes or poor food preparation can allow microbes to grow in food and subsequently infect you. Escherichia coli (E. coli) bacteria sometimes persist in food products such as undercooked hamburger meat and unpasteurized fruit juice. These bacteria can have deadly consequences in vulnerable people, especially children and the elderly.

Cryptosporidia are bacteria found in human and animal feces. These bacteria can get into lake, river, and ocean water from sewage spills, animal waste, and water runoff. Millions can be released from infectious fecal matter. People who drink, swim in, or play in infected water can get sick.

People, including babies, with diarrhea caused by Cryptosporidia or other diarrhea-causing microbes such as Giardia and Salmonella, can infect others while using swimming pools, waterparks, hot tubs, and spas.

Transplanted animal organs may harbor germs

Researchers are investigating the possibility of transplanting animal organs, such as pig hearts, into people. They, however, must guard against the risk that those organs also may transmit microbes that were harmless to the animal into humans, where they may cause disease.

Diagnosing Infectious Diseases

Sometimes your healthcare provider can diagnose an infectious disease by listening to your medical history and doing a physical exam. For example, listening to you describe what happened and any symptoms you have noticed plays an important part in helping your healthcare provider find out what’s wrong.

Blood and urine tests are other ways to diagnose an infection. A laboratory expert can sometimes see the offending microbe in a sample of blood or urine viewed under a microscope. One or both of these tests may be the only way to determine what caused the infection, or they may be used to confirm a diagnosis.

In another type of test, your healthcare provider will take a sample of blood or other body fluid, such as vaginal secretion, and then put it into a special container called a Petri dish to see if any microbe “grows.” This test is called a culture. Lab workers usually can identify certain bacteria, such as chlamydia, and viruses, such as herpes simplex, using this method.

X-rays, scans, and biopsies (taking a tiny sample of tissue from the infected area and inspecting it under a microscope) are among other tools your healthcare provider can use to make an accurate diagnosis.

All of the above procedures are relatively safe, and some can be done in your healthcare provider’s office or a clinic. Others pose a higher risk to you because they involve procedures that go inside your body. One such invasive procedure is taking a biopsy from an internal organ. For example, one way a doctor can diagnose Pneumocystis carinii pneumonia, a lung disease caused by a fungus, is by doing a biopsy on lung tissue and then examining the sample under a microscope.

Treatment of Infectious Diseases

How an infectious disease is treated depends on the microbe that caused it and sometimes on the age and medical condition of the person affected. Certain diseases are not treated at all, but are allowed to run their course, with the immune system doing its job alone. Some diseases, such as the common cold, are treated only to relieve the symptoms. Others, such as strep throat, are treated to destroy the offending microbe as well as to relieve symptoms.

Treatment for Bacteria

The last century saw an explosion in our knowledge about how microbes work and in our methods of treating infectious diseases. For example, the discovery of antibiotics to treat and cure many bacterial diseases was a major breakthrough in medical history. Healthcare providers, however, sometimes prescribe antibiotics unnecessarily for a variety of reasons, including pressure from patients with viral diseases such as the flu.

Because antibiotics have been prescribed too often or prescribed for the wrong diseases for many years, some bacteria have become resistant to the killing effects of these drugs. This resistance, commonly called antibiotic or drug resistance, has become a very serious problem, especially in hospital settings.

Bacteria that are not killed by the antibiotic become strong enough to resist the same medicine the next time it is given. Because bacteria multiply so rapidly, changed or mutated bacteria that resist antibiotics will quickly outnumber those that can be destroyed by those same drugs.

Treatment for Viruses

Viral diseases can be very difficult to treat because viruses live inside your body’s cells where they are protected from medicines in the bloodstream. Researchers developed the first antiviral drug in the late 20th century. The drug, acyclovir, was first approved by the Food and Drug Administration to treat herpes simplex virus infections. Only a few other antiviral medicines are available to prevent and treat viral infections and diseases.

Healthcare providers treat HIV infection with a group of powerful medicines that can keep the virus in check. Known as highly active antiretroviral therapy, or HAART, this treatment has improved and lengthened the lives of many suffering from this deadly infection.

Viral diseases should never be treated with antibiotics. Sometimes a person with a viral disease will develop a bacterial disease as a complication of the initial viral disease. For example, children with chickenpox often scratch the itchy skin lesions (sores) caused by the virus. Bacteria such as Staphylococcus can enter those lesions and cause a bacterial infection. A healthcare provider may then prescribe an antibiotic to destroy the bacteria. The antibiotic, however, will not work on the chickenpox virus, only on the staph bacteria.

Although safe and effective treatments and cures for most viral diseases have eluded researchers, there are safe vaccines to protect you from viral infections and diseases.

Treatment for Fungi

Medicines applied directly to the infected area are available by prescription and over the counter to treat skin and nail fungal infections. Unfortunately, many people have had limited success with them. Very powerful oral antifungal medicines are available only to treat systemic (within the body) fungal infections, such as histoplasmosis. Healthcare providers usually prescribe oral antifungal medicines with caution because all of them, even the milder medicines for skin and nail fungi, can have very serious side effects.

Treatment for Protozoa

Diseases caused by protozoan parasites are among the leading causes of death and disease in tropical and subtropical regions of the world. Developing countries within these areas contain three-quarters of the world’s population, and their people suffer the most from these diseases. Currently, there are no vaccines to control parasitic diseases.

In many cases, controlling the insects that transmit these diseases is difficult because of pesticide resistance, concerns regarding environmental damage, and lack of adequate public health systems to apply existing insect-control methods. Thus, disease control relies heavily on the availability of medicines. Healthcare providers usually use antiparasitic medicines to treat protozoal infections. Unfortunately, there are very few medicines that fight protozoa, and some of those are either harmful to humans or are becoming ineffective.

The fight against the protozoan Plasmodium falciparum, the cause of the most deadly form of malaria, is a good example. This protozoan has become resistant to most of the medicines currently available to destroy it. A major focus of malaria research is on developing a vaccine to prevent people from getting the disease in the first place. In the meantime, many worldwide programs hope to eventually control malaria by keeping people from contact with infected mosquitoes or preventing infection if they can't avoid contact.

Your Immune System's Ability to Fight Infections

Your immune system has an arsenal of ways to fight off invading microbes. Most begin with B and T cells and antibodies whose sole purpose is to keep your body healthy. Some of these cells sacrifice their lives to rid you of disease and restore your body to a healthy state. Some microbes normally present in your body also help destroy microbial invaders. For example, normal, good bacteria, such as lactobacillus in your digestive system, help destroy germs that find their way there.

Other important ways your body reacts to an infection include fever and coughing and sneezing.

Fever

Fever is one of your body’s special ways of fighting an infectious disease. Many microbes are very sensitive to temperature changes and cannot survive in temperatures higher than normal body heat, which is usually around 98.6ºF. Your body uses fever to destroy influenza viruses, for example.

Coughing and sneezing

Another tool in your immune system’s reaction to invading infection-causing microbes is mucus production. Coughing and sneezing help mucus move those germs out of your body efficiently and quickly.

Protection from Infections

We become immune to germs through natural and artificial means. As long ago as the 5th century B.C., Greek doctors noticed that people who had recovered from the plague would never get it again — they seemed to have become immune or resistant to the germ. You can become immune, or develop immunity, to a microbe in several ways. The first time T cells and B cells in your immune system meet up with an antigen, such as a virus or bacterium, they prepare the immune system to destroy the antigen.

Naturally acquired immunity

Because the immune system often can remember its enemies, those cells become active if they meet that particular antigen again. This is called naturally acquired immunity.

Another example of naturally acquired immunity occurs when a pregnant woman passes antibodies to her unborn baby. Babies are born with weak immune responses, but they are protected from some diseases for their first few months of life by antibodies received from their mothers before birth. Babies who are nursed also receive antibodies from breast milk that help protect their digestive tracts.

Artificial immunity

Artificial immunity can come from vaccines. Immunization with vaccines is a safe way to get protection from germs. Some vaccines contain microorganisms or parts of microorganisms that have been weakened or killed. If you get this type of vaccine, those microorganisms (or their parts) will start your body’s immune response, which will demolish the foreign invader but not make you sick. This is a type of artificially acquired immunity.

Immunity can be strong or weak and short- or long-lived, depending on the type of antigen, the amount of antigen, and the route by which it enters your body. When faced with the same antigen, some people’s immune system will respond forcefully, others feebly, and some not at all.

The genes you inherit also can influence your likelihood of getting a disease. In simple terms, the genes you get from your parents can influence how your body reacts to certain microbes.

Preventing Infections

Handwashing

Handwashing is one of the simplest, easiest, and most effective ways to prevent getting or passing on many germs. Amazingly, it is also one of the most overlooked. Healthcare experts recommend scrubbing your hands vigorously for at least 15 seconds with soap and water, about as long as it takes to recite the English alphabet. This will wash away cold and flu viruses and staph and strep bacteria as well as many other disease-causing microbes. It is especially important to wash your hands.....

Before preparing or eating food

After coughing or sneezing

After using the bathroom

After changing a diaper

Medications to Prevent Infections

There are medicines on the market that help prevent you from getting infected by germs. For example, you can prevent getting the flu by taking an antiviral medicine. Vaccines, however, are the best defense against flu viruses. Under specific circumstances, your healthcare provider may prescribe antibiotics to protect you from getting certain bacteria such as Mycobacterium tuberculosis, which causes tuberculosis. Healthcare experts usually advise people traveling to areas where malaria is present to take antiparasitic medicines to prevent possible infection.

Vaccinations to Prevent Infections

In 1796, Edward Jenner laid the foundation for modern vaccines by discovering one of the basic principles of immunization. He had used a relatively harmless microbe, cowpox virus, to bring about an immune response that would help protect people from getting infected by the related but deadly smallpox virus.

Vaccines, however, are not only useful for young children. Adolescents and adults should get vaccinated regularly for tetanus and diphtheria. A vaccine to prevent meningococcal meningitis is recommended for all adolescents. In addition, adults who never had diseases such as measles or chickenpox during childhood or who never received vaccines to prevent them should consider being immunized. Childhood diseases can be far more serious in adults.

If you travel or plan to travel outside the United States, getting the immunizations that are recommended for your destination(s) is very important. Vaccines can prevent yellow fever, polio, typhoid fever, hepatitis A, cholera, rabies, and other diseases that are more prevalent abroad than in the United States.

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